1. Field of the Invention
The present invention relates to a Braun tube discharge apparatus, and relates especially to a Braun tube discharge apparatus that includes a so-called spot killer function.
2. Description of the Related Art
A conventional Braun tube discharge apparatus that includes a spot killer function, such as is shown in FIG. 5, is disclosed in Japanese Patent Unexamined Publication No. Hei. 10-248019.
In FIG. 5, a vertical pulse signal, obtained by the performance of a synchronous separation operation at a one-chip IC 1, is output via a vertical drive signal output terminal 1a, and is input to a vertical deflection IC 2 via a vertical trigger input terminal 2a. Upon receiving the vertical pulse signal, the vertical deflection IC 2 generates a saw-toothed voltage signal that it outputs via a vertical amplitude control terminal 2b. The saw-toothed signal is employed to drive an electron beam vertically. As for the saw-toothed signal, the side above a predetermined center potential corresponds to the scan location in the upper portion of the screen, and the side below the predetermined center potential corresponds to the scan location in the lower portion of the screen.
Upon receiving a power-OFF instruction, the one-chip IC 1 outputs a spot killer command signal via a spot killer command terminal 1b, and upon receiving this command signal, a spot killer circuit discharges, within a short period of time, a residual electrical charge that is held by the anode of a Braun tube. When the electrical charge that is discharged strikes the screen, the screen glows momentarily. At that time, when an impact point is fixed, an electrical charge will concentrate on that point every time the power is turned off, and a burn in the Braun tube will occur. Therefore, even when the power-OFF instruction is issued, the vertical deflection IC 2 is activated to validate the vertical drive, so that the electrical charge can be dispersed vertically when it is discharged.
However, if only the vertical drive is validated, a lightning-shaped flash of light will be displayed vertically on the screen. This may frighten a user, or the user may assume that a specific failure has occurred. Therefore, in the discharge apparatus shown in FIG. 5, the path between the collector and the emitter of a transistor Q1 is rendered conductive in accordance with the output of the spot-killer command signal, and a vertical pulse signal transmission path is grounded. Then, the voltage input at the vertical trigger input terminal 2a goes to a low level and the scan location is moved to the lower portion of the screen. On the other hand, when the vertical pulse signal transmission path is grounded, the base voltage of a transistor Q2 is changed, so that the path between the collector and the emitter of the transistor Q2 is rendered conductive, and the emitter potential is drastically reduced. Accordingly, there is a drastic increase in the amplitude of a saw-toothed signal, and the length of the vertical axis of the screen is increased. That is, when the residual electrical charge is discharged from the anode of the Braun tube, the scan location is moved to the lower portion of the screen, so that it is difficult for a user to recognize it. Further, since the length of the vertical axis of the screen is increased, even when a point on the screen glows, the light-emitting point is moved outside the screen frame, and the lightning-shaped flash of light is not displayed.
According to a Braun tube discharge apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 5-191753, when the power is switched off, the screen is turned off at a constant timing in synchronism with a vertical sync signal. In this publication, it is taught that in this manner the quality for the turning off of the screen can be improved.
The following problem is encountered in the above conventional apparatuses.
Since it is presumed that the first conventional apparatus is employed for the vertical deflection IC of a vertical pulse drive type or the like, this apparatus can not be employed for a vertical deflection IC of a so-called vertical ramp drive type or the like. This is because when a vertical deflection IC of the vertical ramp drive type receives a vertical ramp drive signal, it returns part of the output as negative feedback input, performs differential amplification of the negative feedback input, and outputs the result. Even when grounding of the path for the input of the vertical ramp drive signal is synchronized with the spot killer circuit, the same effects can not be obtained.
For the second conventional apparatus, although the screen is turned off in accordance with a constant timing, the above-described arrangement can not prevent the display of a flash of light.